Darkstar 12" f/6 dob restoration

[JamesF]

As I posted previously I am now the keeper of a 12" f/6 Darkstar dob previously owned by Ade King of this parish, and as we've both posted it's in need of a reasonable amount of work to get it usable again.  Having left it to loom over the power tools in the workshop for a few days I thought I'd start with a summary of things that need doing and perhaps a few improvements.

The first thing to fix is the secondary assembly.  This is what remains of it:

At some point the original secondary mirror appears to have taken a swan dive off it's mount and crashed into the primary below, getting badly damaged in the process.  One of the spider arms has broken free of its retaining weld (perhaps it wasn't a very good weld in the first place because the arms appear to be cut from a sheet of galvanised steel).  The arm on the right is also missing the retaining nut (another weld failure, most likely).  There's what appears to be a screw clamping the secondary height adjuster in place, but as things turn out it looks to be a bit of threaded bar with a nut welded onto the end (which has also failed).  The mirror mount itself (a piece of plastic tube cut at 45 degrees) I do not intend to re-use.  I don't feel it provides a large enough area for fixing the mirror in place.

A kind SGLer (@andrew s I think Ade said?) donated a new secondary to the project, currently buried in cotton wool.

And Ade has made up a bit of aluminium to fix it to.

The rest of the assembly I'm still debating over.  I could strip back the paint and re-weld the spider arm and a new nut, and fix up the clamp screw.  Another SGL member also donated a somewhat unusual-looking spider:

I could use that instead, but it's actually a little short so I'd need spacers and I think that might make a bit of a mess with diffraction spikes.

Another possibility is to cut the remaining arms off and fit them to a piece of round aluminium bar that has one end cut at 45 degrees, similar to the way that Skywatcher do it on their OTAs:

then fix threaded rods to the other ends of the arms and use knurled wheels on the outside of the tube as adjusters.

The finder is an "interesting" bit of kit, made from a binocular objective (and perhaps eyepiece as well?)

The tube forming the main barrel of the finder has a neat little set of crosshairs made in the end.

I assume they have to be at the focal plane of the primary lens otherwise they're not in focus.  They eyepiece itself has some odd sort of seal around it that is badly perished.  Or very well perished, depending on how you look at these things.

I'm quite taken by the Heath-Robinson engineering, but I can't help thinking that a standard 50mm finder would just be so much simpler.  Perhaps a right angle model, given the size of the OTA.  It has also crossed my mind that I could perhaps fit something like an ST80 as a finder.  On a scope of this size that might work fairly well.

The coating of the primary mirror has some damage from the secondary hitting it, but for the moment all I propose to do is to give it a gentle wash.

I'm not hugely wild about the plywood cell and mirror clips, but for the time being I think they can stay.  Once I have it all back together and usable I'll have another think about that.

And the last thing I think needs immediate attention is the focuser.  The existing one is a bit basic, to say the least.

A dig around in the observatory came up with this as a possible replacement (which would also allow 2" eyepieces to be used).

It needs a new slow-speed knob, but I can probably 3d-print one, or turn one from a piece of brass.  I think I'd also need to 3d-print a base to fit the tube that I could fit this onto, but that would at least prevent light leakage from the sides.  Someone appears to have done that with the existing one by filling the gaps with something like blutak.

The other thing Ade gave me was this:

A dob knob.  No self-respecting dob should be without one.

Other things I need to think about are probably a barrow for moving it around, as whilst it's not particularly heavy the size does make it a little awkward, and some sort of restraint or lock to stop the tube moving in storage (there's part of a chain still fixed to the rocker box that might have been for that purpose).  Oh, and a pair of platform sole observing boots, so I can reach the focuser when the OTA is pointing at the zenith

For the time being I think that's it.  There are other ideas for improvements that were discussed in Ade's thread, but I think returning it to use should be the first target.  The focuser and a 50mm finder shouldn't be too hard to sort out so I may well start there whilst I cogitate on the issue of the secondary assembly a little further.  The mirror I can deal with last.  I'll also keep my eye out for anything along the lines of an ST80 on AB&S.

James

Edited February 25, 2020 by JamesF

[AdeKing]

Thanks for the update James, I'm really pleased to see that you've started out with great enthusiasm and it sounds like you'll have the drive to get it finished.

I actually found the damaged secondary on Monday night whilst I was rummaging around one of my parts boxes, why I'd kept it I have no idea.

It is very much Heath Robinson and it looks so much like the DIY Dobs you see so many of on USA threads.

Good luck and I'll be watching with interest.

[Piero]

Good luck with the restoration of this Dobson. Hopefully it will see some light soon! 

[Bruce Leeroy]

Regarding the existing finder you should be able to push fit a 0.9 inch mirror diaganol straight into it if it's the same as mine. I seem to remember having to reduce the length of the barrel a little to gain focus.

[JamesF]

I removed the focuser today.  Looks like the stuff used to fill the gaps around it was indeed blutak.

The existing hole for the drawtube measured just over 50mm, so I assume it's 2".  For the new focuser I need at least 60mm and 65mm would probably be better.  I'm sure I have a holesaw in that kind of size range so I think I'll fix some scrap timber to the inside of the OTA using screws through the existing mounting holes and use that to centre the saw whilst I enlarge it.  The mounting holes are about 82mm across the diagonals so I think there should be enough clearance.

I also measured the outside circumference of the tube (1122mm) which gives an outside radius of about 178.6mm, so now I know what I need to match when I design the mounting block for the new focuser.

James

[JamesF]

A quick trip to the workshop reveals that I have a 65mm holesaw and the next size up is 72mm.  Either would suit the new focuser, but I think I'll try to keep to the smaller opening for now.

James

[JamesF]

Yesterday I 3d-printed a block to go between the focuser and tube which should block stray light entering from outside.

I checked the curve against the tube and it's a close fit so I clearly measured sufficiently accurately

Assuming I can find some bolts of a suitable length I'll try to get it fitted this weekend.  It doesn't look as though there's going to be opportunity for doing much else that doesn't involve hiding from the weather

James

[Chriske]

I'm busy doing exactly the same, together with my son.
Also a Darkstar 12" f/6.
Think we're going to dump the tube and the(to) heavy Dob and make a truss instead.

[JamesF]

Well, I didn't manage to find any suitable bolts, so they're ordered and on the way.  I did however screw a block of scrap timber to the inside of the OTA

and cut a larger diameter hole for the new focuser.

I didn't quite get it perfectly centred on the old hole, but that shouldn't make any difference.

James

[JamesF]

The new stainless bolts arrived earlier this week but I've had little chance to get out to the workshop, so this morning I got out and started work on fitting the new focuser.  This took a little longer than planned as I realised halfway through that I would have to remove the focuser body from the backplate to get access to the lower two holes on the backplate in order to mark up the tube for drilling the new mounting holes (and actually it makes fitting easier, too).  There are four grub screws in the edges of the backplate that can be slackened off to allow the focuser body to be rotated and backing them out a little further allows it to come away completely so it's not hard, but for the fact they seemed to be done up exceptionally tight to start with.  The tiny Allen key I had to use was flexing rather more than I was entirely comfortable with before they began to turn.

Anyhow, all sorted now.

James

[JamesF]

I think the next step has to be sorting out the spider and secondary mirror assembly.

Now I've had some time to consider options, what I'd really like to do is to build a secondary assembly similar to those used on more modern newtonian models, using two pieces of round aluminium bar (one cut at 45 degrees for the mirror mounting).  TS sell such items though they're far from cheap so I'll have a go at making one myself.  I can then mount the plate that Ade already made on the end and fit the mirror to that.

James

[Chriske]

I'm a bit surprised you did not print a focuser...

[JamesF]

3 hours ago, Chriske said:

I'm a bit surprised you did not print a focuser...

I have a drawer full of spares from replacing them on other telescopes.  No need to make things harder than they need be :)

James

[AdeKing]

On 07/03/2020 at 15:37, JamesF said:

I think the next step has to be sorting out the spider and secondary mirror assembly.

Now I've had some time to consider options, what I'd really like to do is to build a secondary assembly similar to those used on more modern newtonian models, using two pieces of round aluminium bar (one cut at 45 degrees for the mirror mounting).  TS sell such items though they're far from cheap so I'll have a go at making one myself.  I can then mount the plate that Ade already made on the end and fit the mirror to that.

James

Beacon Hill telescopes were still producing parts a couple of years ago and for a reasonable cost when I spoke to Barry Watts after first getting the Dark Star monster, but I never followed up.

http://beaconhilltelescopes.org.uk/

Prices on the website are very old, but when I spoke to Barry he said the secondary spider wouldn't be much different in price.

Just food for thought if you want an easy life.

Ade

[JamesF]

Thanks Ade.  I might well look into that, though I did discover a lump of 42mm diameter round aluminium bar in the workshop at the weekend that might require me to have a crack at it first :D

James

[fozzybear]

On ‎26‎/‎02‎/‎2020 at 00:35, JamesF said:

The rest of the assembly I'm still debating over.  I could strip back the paint and re-weld the spider arm and a new nut, and fix up the clamp screw.  Another SGL member also donated a somewhat unusual-looking spider:

that is an old OO spider circa 2000 I had one for my 8" Dob

[JamesF]

46 minutes ago, fozzybear said:

that is an old OO spider circa 2000 I had one for my 8" Dob

Useful to know where it comes from.  I may at some point be able to pass it on to someone else in need.

James

[AdeKing]

4 minutes ago, JamesF said:

Useful to know where it comes from.  I may at some point be able to pass it on to someone else in need.

James

I think Shane told me that the spider  came from an old OO UK 12" Dob, but the monster Dark Star 12" has a rather ........erm........oversized tube, being about 16" diameter I seem to remember.

[JamesF]

3 minutes ago, AdeKing said:

I think Shane told me that the spider  came from an old OO UK 12" Dob, but the monster Dark Star 12" has a rather ........erm........oversized tube, being about 16" diameter I seem to remember.

Given that my measurement of the outside diameter of the tube appears to have been correct at 1122mm, I think that works out as just over 14" diameter.  It does look enormous though.  A 250mm Skywatcher dob is the biggest I'd ever seen in the flesh beforehand and it would be completely dwarfed by the Darkstar.

James

[Chriske]

In those days we thought it was 'normal', but these big DarkStar scopes are far to heavy imo.
We're going to dump the tube and will install trusses.

[JamesF]

To be honest, I don't think my tube is that heavy at all.  I didn't find it awkward to lift out of the car and Ade appeared to carry it a fair distance from his garage without struggling (though perhaps he was just good at covering it up :)

Quite likely it will be outweighed by the primary (plus cell), the focuser and finder etc.  The base is quite possibly the heavier part.  It could quite possibly be slimmed down with a few holes drilled here and there just to remove material without really compromising the strength and rigidity, but I don't really feel the need at the moment.

Perhaps yours is not the same construction as mine?  What I have looks to be some form of plastic about 2.5mm thick (from memory -- I've not measured it).

James

[AdeKing]

27 minutes ago, JamesF said:

Given that my measurement of the outside diameter of the tube appears to have been correct at 1122mm, I think that works out as just over 14" diameter.  It does look enormous though.  A 250mm Skywatcher dob is the biggest I'd ever seen in the flesh beforehand and it would be completely dwarfed by the Darkstar.

James

Actually, 14" sounds more like it, approx 2" bigger than the mirror diameter, so about 1" to spare all round the mirror.

1 minute ago, JamesF said:

To be honest, I don't think my tube is that heavy at all.  I didn't find it awkward to lift out of the car and Ade appeared to carry it a fair distance from his garage without struggling (though perhaps he was just good at covering it up

Quite likely it will be outweighed by the primary (plus cell), the focuser and finder etc.  The base is quite possibly the heavier part.  It could quite possibly be slimmed down with a few holes drilled here and there just to remove material without really compromising the strength and rigidity, but I don't really feel the need at the moment.

Perhaps yours is not the same construction as mine?  What I have looks to be some form of plastic about 2.5mm thick (from memory -- I've not measured it).

James

No, I wasn't putting on an act James, you're right about it not being too heavy, I was actually pleasantly surprised by how light it was when I first collected it.  Much lighter than the size of it suggests.

I just found it a bit cumbersome to carry, mainly due to the OTA length. 

If the OTA had a more comfortable handle fitted than the thin metal cupboard handle it came to me with, then I'd go as far as to say that it would be fairly comfortable to carry weight wise.

[AdeKing]

24 minutes ago, Chriske said:

In those days we thought it was 'normal', but these big DarkStar scopes are far to heavy imo.
We're going to dump the tube and will install trusses.

I was going to do that with this one when it was in my custody, but mainly to make it easier to transport as I only have a small hatchback car.

In the end I didn't have time to dedicate to it, hence how it came to be in James's possession.

[fozzybear]

39 minutes ago, JamesF said:

Given that my measurement of the outside diameter of the tube appears to have been correct at 1122mm

are you talking about tube length as that would be 44 inch's and that would be a beast if diameter

[JamesF]

2 minutes ago, fozzybear said:

are you talking about tube length as that would be 44 inch's and that would be a beast if diameter

Not diameter, sorry :)

I was thinking about obtaining the diameter and my brain got ahead of my fingers.  1122mm is the outside circumference of the tube.

James